Usually a .357 SIG barrel designed for a particular model will fit in the .40 S&W version of that same model. So a P229 barrel will only fit a P229, etc. With the P239, a 9mm barrel will also fit in the .40/.357 slide, but that is not the case for most SIGs.
If you post a pic of it we can probably figure out what it's for. Certain guns only come in certain calibers, and each model has a specific barrel length. See if you can get a pic with a ruler next to it.
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